1. Field of the Invention
The present invention relates to a radio communication device, a radio communication method and a non-contact IC card reader/writer device for transmitting information to a radio communication medium such as a non-contact IC card and a cellar phone and receiving information from the radio communication medium.
2. Description of the Related Art
In recent years, a con-contact IC card available in non-contact has been widely used in terms of a security of information and a convenience in carrying. A technique in relation to a non-contact information/reception using this kind of non-contact IC card is disclosed by, for example, Jpn. Pat. No. 3,579,899 (Jpn. Pat. Appln. KOKAI Publication No. 8-194785).
For example, a reader/writer provided with a single loop antenna shared in transmission and reception is known. A resonant frequency of this kind of loop antenna shared in transmission and reception is uniquely decided by a capacity (circuit constant) of a capacitor connected to this loop antenna.
A control unit and the loop antenna of the reader/writer are connected with each other by a cable with two lines through a connector. Thereby, the reader/writer can communicate with the IC card through the loop antenna.
An object of the above mentioned reader/writer is to adjust the antenna resonant frequency by adjusting a value of the capacitor connected to the loop antenna shared in transmission and reception and secure a stable communication property and a wide communication area (excellent communication property in assumed and prescribed communication area) with the IC card.
Further, a reader/writer provided with two loop antennas independent for a transmission and a reception, respectively, is known. The resonant frequency of this kind of loop antenna for a transmission is decided by the capacity of the capacitor connected to. And the resonant frequency of this kind of loop antenna for reception is decided by the capacity of the capacitor connected to the loop antenna.
Both of the forgoing two reader/writers for a transmission and a reception have their objects to adjust the antenna resonant frequencies and secure the stable communication properties and the wide communication areas (excellent communication properties in assumed and prescribed communication areas) with the IC cards.
The above-described reader/writer can be compatible, by fixed antenna resonant frequencies, with such specific IC cards in a traffic system for a station operation and an electronic money system and for IC cards different in resonant frequency on IC card sides within narrow ranges (extent to several dozen kHz) and can obtain an excellent communication property.
However, the expand use of IC cards has brought about the following situations.
(i) IC cards made by and issued from a plurality of manufactures and issuers, respectively, are used for the same system.
(ii) Shared IC card are used for a plurality of systems.
In this situation, we are faced to a new problem to be solved. Because of the antenna resonant frequency on the reader/writer side is fixed to the IC cards having largely different antenna resonant frequencies (several hundred kHz-several MHz), such a reader/writer cannot obtain performance (excellent communication property) for an IC card (b) having a different resonant frequency fb even by adjusting an antenna circuit constant, a resonant frequency, etc., thereof so as to make the property excellent for an IC card (a) having a resonant frequency fa. In contrast, in the case of an adjustment on the reader/writer side for the IC card (b), since the IC card (a) is deteriorated in communication property, it is hard for the reader/writer to set both IC cards in order to get excellent properties.
The reason of the above-mentioned problem comes from the fact that the card antennas having different resonant frequencies with each other causes mutual induction with the loop antenna of the reader/writer to cause strong coupling by making both loop antennas for a and a reception be resonant antennas to have the resonant frequency, respectively, and an area in which reception signals from the IC cards are suppressed and brought into a voltage level impossible to demodulate them is generated. As a result, the communication performance of the reader/writer is deteriorated.